Microelectronic devices, such as semiconductor devices, are generally fabricated on and/or in wafers or workpieces. A typical wafer plating process involves depositing a seed layer onto the surface of the wafer via vapor deposition. The wafer is then moved into an electroplating processor where electric current is conducted through an electrolyte to the wafer, to apply a blanket layer or patterned layer of a metal or other conductive material onto the seed layer. Examples of conductive materials include permalloy, gold, silver, copper, and tin. Subsequent processing steps form components, contacts and/or conductive lines on the wafer.
In some electroplating processors, a current thief electrode, also referred to as an auxiliary cathode, is used to better control the plating thickness at the edge of the wafer and for control of the terminal effect on thin seed layers. The terminal effect for a given seed layer increases as the electrical conductivity of the electrolyte bath increases. Hence, a current thief electrode can be effectively used with thinner seed layers combined with high conductivity electrolyte baths. The use of thin seed layers is increasing common with redistribution layer (RDL) and wafer level packaging (WLP) plated wafers. For example, it is expected that RDL wafers may soon have copper seed layers as thin as 500 A-1000 A and copper bath conductivities of 470 mS/cm or higher.
In WLP processing, a relatively large amount of metal is plated onto each wafer. Consequently, in a WLP electrochemical processor having a current thief electrode, a large amount of metal will also be plated on the current thief electrode. This metal must be deplated or otherwise removed from the current thief electrode at frequent intervals, with the processor removed from use during the deplating operation. Deplating the current thief electrode can also result in contamination particles in the electrolyte bath.
Damascene electroplating processors have used a current thief electrode, in the form of a platinum wire, inside of a membrane tube. The membrane tube holds a separate electrolyte (referred to as thiefolyte) having no metal (e.g., a 3% sulfuric acid and deionized water solution). The thief cathode reaction mostly evolves hydrogen rather than plating copper onto the wire. The hydrogen is swept out of the tube by the flowing thiefolyte. However, some metal does cross the membrane into the thiefolyte and plates onto the platinum wire (especially when using a lower conductivity bath). Consequently, the thiefolyte is only used once and flows to drain after passing through the membrane tube. The platinum wire is deplated after processing each wafer. However, under certain conditions using high thief current, it may be difficult to fully deplate the platinum wire.
The amp-minutes involved in processing RDL and WLP wafers can be 20 to 40 times higher than for damascene. As a result, the wire in a membrane tube thief electrode used in damascene electroplating may not suitable for electroplating RDL and WLP wafers, due to excessive metal plating onto the thief electrode wire, and excessive consumption of thiefolyte. Accordingly, engineering challenges remain in designing apparatus and methods for electroplating RDL and WLP wafers, and other applications, using a thief electrode.